1. Technical Field of the Invention
The present invention is directed to a fast-acting, high-flow valve. In particular, the invention relates to a valve with an annular valve plug and an annular valve seat.
2. Description of the Related Art
A very fast-acting valve is desirable in many applications including aircraft control systems, pulsejet engines, and chemical and pharmacological processes. Desirable features of a good fast-acting valve include: minimal leakage in the closed state, rapid switching between the closed state and the fully open state, accurate definition of closed and open states, short cycle time for repetitive applications, and large fluid flux through the valve in the open state.
Meyer describes a fast-acting valve in U.S. Pat. No. 4,344,449. In this valve, a specially shaped valve stem acts as a sliding gate to the pressurized fluid. An electromagnetic actuator drives the valve stem axially, which opens the valve. A sealed air chamber cooperates with the specially shaped valve stem to form a nonlinear gas spring that helps to return the valve to its closed state. This approach results in a rapid release of a short blast of pressurized gas. However, to reduce friction, sliding gate valves typically have small contact pressures, which leads to leakage when the differential fluid pressure across the valve is large.
A fast-acting high-output valve is disclosed by Jaw et. al. in U.S. Pat. No. 5,485,868. This valve comprises a series of pieces that are separated by motion guards and which are arranged so that all of the pieces come together at a common central location. The edges and radial periphery of each piece seal against a valve seat when the valve is closed. A hinge is provided for each piece such that a downward actuating force at the common central location causes the periphery of each piece to move upward, thereby providing an opening for fluid to flow. Concerns about the possibility of substantial leakage with this valve design motivated a continued search for an appropriate fast-acting valve.
A review of the various valve designs discussed by Burmeister, Loser and Sneegas in xe2x80x9cNASA Contributions to Advanced Valve Technologyxe2x80x9d (NASA SP-5019) revealed no designs that satisfactorily achieved all of the desirable features of a fast-acting valve.
Although a well-designed plug valve could eliminate the leakage problem, a standard plug valve is difficult to open rapidly without long-term adverse effects. For instance, for a valve with a 100 mm2 orifice area, a circular valve seat will have a diameter slightly greater than 11 mm. The fully open valve state requires that the gap between the valve plug and the valve seat be about half the distance between the edges of the valve seat, or approximately 5.5 mm in this case. To move the valve plug from a closed state to the fully open state in 0.5 ms requires the valve plug to have an average speed in excess of 10 m/s, thereby requiring an acceleration greater than 4000 g (where g is Earth""s acceleration of gravity) during the valve opening. Such a large acceleration is difficult to achieve for a large number of cycles without inelastic deformation.
The present invention seeks to overcome the difficulties associated with prior valves by using an annular plug valve. The annular plug valve includes an annular valve seat that defines an annular valve orifice between the edges of the annular valve seat, an annular valve plug sized to cover the valve orifice when the valve is closed, and a valve-plug holder for moving the annular valve plug on and off the annular valve seat. The use of an annular valve orifice reduces the characteristic distance between the edges of the valve seat. Rather than this distance being equal to the diameter of the orifice, as it is for a conventional circular orifice, the characteristic distance equals the distance between the inner and outer radii (for a circular annulus). The reduced characteristic distance greatly reduces the gap required between the annular valve plug and the annular valve seat for the valve to be fully open, thereby greatly reducing the required stroke and corresponding speed and acceleration. Although annular valve seats and plugs have been used previously (for instance, see the concentric disk valves shown in FIG. 14.3.8 of Mark ""s Standard Handbook for Mechanical Engineers, Ninth Edition, McGraw-Hill Book Company, New York), their use has been confined to check-valve applications. In a check valve, opening and closing is not controlled directly, rather differential fluid pressures on opposing sides of the valve plug are responsible for opening and closing the valve. In the current invention, a valve-plug holder that is independently controlled moves the annular valve plug to open and close the valve. The independently controlled opening and closing of the valve is important for achieving controllable fast operation of the valve.
The annular plug valve requires a suitable actuator for imparting movement to the annular valve plug. Actuators that impart movement through an impact are desirable for this application because maximum velocity is reached over a very short time interval. A variety of impact actuators have been devised to meet the needs of the annular valve plug. These actuators comprise a shaft that is impacted at one end by an impactor. The acceleration of the shaft takes place only over the time interval in which the impactor maintains contact with the end of the shaft. The shaft achieves its maximum velocity at the end of the impact interval, which is very short. The ability to reach maximum velocity in a very short time interval is highly desirable for the valve actuator.
To close the annular plug valve rapidly, a short braking stroke is required. The dissipation of kinetic energy associated with the motion of the valve in a short braking stroke represents difficulties for known damping devices. Hence, a method of shock braking was devised. A shock brake can be considered as a spring with large internal damping, which is achieved by friction between bodies. This process requires first and second bodies to have locally substantially parallel contact surfaces that are inclined to a translation direction. Preferably the first body is shaped as a truncated cone and the second body is annular. As the first body translates axially, it impacts the second body. The bodies deform elastically. Mutual sliding of the respective contact surfaces occurs. During the sliding, frictional forces dissipate much of the kinetic energy. The remainder of the energy is stored in the elasticity of the bodies, most of which is dissipated frictionally as the bodies return to their original shapes. Multiple additional bodies can be used to enhance the performance of the shock brake.